1. Field of the Invention
The present invention relates to the field of analytical chemical instrumentation utilizing optical sensors and in particular to integrated optical-chemical analytical instrumentation used in the fields of chemical, biochemical, biological or biomedical analysis, process control, pollution detection and control, and other similar areas.
2. Discussion of the Related Art
Referring to FIG. 1, a prior art optical sensor 100 is shown. The optical sensor 100 utilizes surface plasmon resonance (SPR). The electro-optic components of the optical sensor 100, including light emitting diode 120, photodetector array 110, and temperature sensor 125, may be encapsulated within a trapezoidal-shaped optical housing 150 and coupled to an interior surface 161 of a substrate 160. A plurality of conductive leads 165 are coupled to an exterior surface 162 of the substrate 160. An optical window 140 made from glass is attached to the optical housing 150 to form part of sensing surface 145, which includes a gold (Au), silver (Ag) or copper (Cu) metal thin film 145 deposited onto a surface of the glass optical window 140. The layer 145 is preferably planar although other configurations, such as convex or concave configurations, or featured with steps, periodic or non-periodic, can also be utilized. This layer 145 may comprise a Au film approximately 175 Å thick. The thickness of the Au layer may vary from about 175 to about 600 Å and still permit SPR to occur. The specific film thickness is determined by the frequency of the radiation for the light source 120 and the properties of the conductive material used for layer 145.
Optical housing 150 has an optical geometry such that light from a light emitting diode (LED), solid state laser or other appropriate light source 120 will reflect from sensing surface 145 to mirrored surface and then strike photodetector 110. Light source 120 may comprise a LED, laser diode, light filament, halogen lamp, or other suitable source of electromagnetic radiation. In one embodiment of the prior art, a plurality of light sources that emit light of different wavelengths may be used. The photodetector 110 is multi-channeled and may be linear or two-dimensional. Other configurations of optical housing 150 may be employed consistent with optical sensor 100. For example, light from LED 120 may reflect from mirrored surface to sensing surface 145 and then strike a photodetector 110.
Optical housing 150 is made of a light transmissive material in which light 127 from light source 120 travels. Suitable materials include glass, plastic or hardened epoxy, although other materials may be used that preferably will not damage the encapsulated components. In particular, an epoxy marketed under the trademark Epocast.RTM. 2013 Parts A/B by Furane Products Company has been found useful, especially for radiation sources in the infrared range. Other usable materials include Emerson & Cumming, Stycast 1269A Parts A/B, Tracon Trabond F114, Dexter Hysol OS1000, Norland 61 and 63, Dexter Hysol MG18, and Nitto 8510-1100.
Optical housing 150 is coupled to the substrate 160 to form an encapsulated self-contained sensor 100. The substrate 160 may be made of a dark, light-absorbing material, such as a hard resin or epoxy. However, the material of substrate 160 depends primarily on the radiation properties of light source 120. Also, substrate 160 may be coated with a dark layer of light-absorbing material such as polyurethane epoxy or a thin resin layer among others.
Temperature sensor 125 may also be embedded within housing 150 and coupled to interior surface 161 of substrate 160. It is desirable that temperature sensor 125 be disposed as close to the sensing surface 145 as is practical. A polarizer 121 may be used to produce transverse magnetic polarized light (the electric field polarized in a plane of incidence being the sensing surface 145) from the light source 120. A filter (not shown) may also be used to screen out radiation at wavelengths other than wavelengths produced by light source 120. This filter may overlay photodetector 110 and serves to pass radiation at the wavelengths produced by light source 120 to photodetector 110. As such, the filter eliminates unwanted noise caused by other radiation sources in proximity to the sensor 100. One suitable filter is a plastic filter material marketed by Polaroid Corporation known as XR-84. This material is especially suitable for passing infrared radiation and blocking visible radiation.
An alternative to utilizing a filter is to utilize a plastic or epoxy material for the housing 150 which is transparent to wavelengths produced by the light source 120 and opaque to frequencies outside the desired frequency range of interest for a given sensor/sample combination. Likewise, an absorbing die can be enclosed in the housing 150 to achieve the same function.
Those of skill in the art will recognize that the elements of sensor 100 can be relocated, or rearranged about the sensor substrate 160 while retaining equivalence in function according to the invention. For example, mirrored surfaces utilized for reflecting the light rays could take on other configurations and locations within the sensor 100 so long as the light strikes the sensing surface 145 and the intensity of the radiation reflected therefrom is measured as a function of the angle of the radiation striking the sensing surface 145. Photodetector array 110 receives the light incident over a broad range of angles and yields a voltage output for each light cell where sufficient light is sensed. The output of each cell can be carried on interface 165, as individual binary signals of each photo cell, to an external system or component (not shown), such as a DSP, PC104-based microprocessor, hand-held meter, calculator, printer, logic analyzer, oscilloscope, or other similar system.
Referring to FIG. 2, the well established optical geometry for SPR is illustrated. The angle (θ) at which SPR occurs is highly dependant on the refractive index of the material in contact with the thin metal film 145 deposited on the dielectric 140. As is known in the art, when radiation strikes a thin conductive film at the interface of an insulator, the intensity of reflection thereof is a function of the angle of incidence of the radiation onto the film and the refractive index of the material in contact with the other side of the film. Hence, by determining the angle at which minimum reflectance occurs, it is possible to determine the index of refraction of the material on the side of the film opposite the side the radiation is reflected from. For a given wavelength of the incident light, resonance occurs at a specific angle of incidence that is dependent on the index of refraction of material in contact with the thin metal film. Therefore, changes in the index of refraction of the material in contact with the thin metal film result in changes in the SPR angle.
Miniaturized SPR sensors are becoming available for use in some biochemical applications, but their overall usefulness in other applications is limited. Specifically, the direct detection of metal ions in liquids by a miniaturized, cost-effective, accurate sensor is not known to exist. Currently, for most applications, highly accurate, reliable metal ion concentration analysis is restricted to laboratory scale measurements made ex situ and off site from grab samples. Metal ion concentration systems that may be used “in the field,” are expensive, have slow response times, and are large and bulky—essentially expensive and inconvenient. What is needed is a miniaturized, cost-effective, accurate sensor for use in situ for the direct detection of metal ions in liquids.